Teaching Case

Persistent left proximal hip pain for three months. Tender on examination. No history of trauma.

X-ray of the pelvis shows a well defined lucency within the metaphysis of the proximal femur. It has a thin sclerotic margin and contains internal ossification. There is no periosteal reaction or pathological fracture.

MRI confirms a 1.7cm intramedullary lesion which is isointense to muscle on T1 and high on T2 with central low signal calcific foci. A small amount of T1 high signal is seen at the superior aspect of the lesion in keeping with fat. There is no evidence of cortical destruction and no soft tissue component is seen.

Liposclerosing Myxofibrous tumour of bone (LSMFT) is a fibro-osseous lesion that is characterised by a complex mixture of histological elements. The location in the intertrochanteric region of the femur is the key element to consideration of this diagnosis¹.

Histologically it is characterised by a mixture of elements including lipoma, fibroxanthoma, myxoma, myxofibroma, fibrous dysplasia like features, cyst formation, fat necrosis, ischaemic ossification and rarely cartilage².

The origin of LSMFT is unclear. The most common theories suggest it is a variant of fibrous dysplasia or it represents degenerative change in benign entities such as lipomas, simple bone cysts, fibromyxomas or bone infarcts ^1,2,3.

LSMFT most commonly presents with pain but has been discovered as an incidental finding¹. It can also present as a pathological fracture. It usually occurs in the fourth to sixth decades with an equal distribution between men and women. Whilst it is generally considered benign, there is a well documented risk of malignant transformation of between 10-16% ^1,2,3,4. Transformation into osteosarcoma, fibrosarcoma and malignant fibrous histiocytoma has been demonstrated¹. Malignant transformation has been linked to lesions containing necrotic fat with secondary calcification and reactive ossification with ischaemic woven bone⁴. It has been suggested that malignant transformation may occur within the reactive border of ischaemic bone or arise from atypical lipomatous elements ^1,4,5.

The role of the radiologist is to make the initial diagnosis and exclude other differentials. It is most specifically characterised by its location with over 90% of lesions occurring in the central metadiaphysis of the proximal femur. The imaging features are non-aggressive with the classic description being that of a lucent lesion with a narrow zone of transition, thin sclerotic border and internal mineralised matrix. On MRI it is isointense to muscle on T1 and predominantly high signal on T2 with areas of low signal corresponding to the mineralised matrix.

The significant risk of malignant transformation in LSMFT necessitates the need for close observation. Biopsy should be performed on patients who are symptomatic or who have deteriorating symptomatology. Asymptomatic patients are followed up with serial MRI.

In summary Liposclerosing Myxofibrous Tumour of Bone is a complex histological lesion with characteristic radiological findings and a strong predilection for the intertrochanteric region of the femur. The significant risk of malignant transformation necessitates the need for close observation of this lesion¹.